Impedance Matching for Ultrasound Phased Array Elements

ABSTRACT

A system includes a transducer configured to emit ultrasound in response to receiving an electrical signal from a driving circuit. The transducer includes a first dimension that determines a frequency of the ultrasound and a second dimension that determines an impedance of the transducer. The frequency is independent of the second dimension.

CROSS-RELATED APPLICATION

Under 35 U.S.C. 119(e)(1), this application claims the benefit ofprovisional application Ser. No. 60/736,713, filed Nov. 15, 2005.

TECHNICAL FIELD

This invention relates to therapeutic and diagnostic applications ofultrasound, and more particularly to controlling the electricalimpedance of an ultrasonic transducer for use in an ultrasound phasedarray.

BACKGROUND

A phased array includes an array of ultrasound transducers, each ofwhich produces ultrasound in response to receiving a control signal fromradio frequency (RF) driving circuitry. The RF driving circuitryelectronically adjusts the phase, and amplitude of the ultrasoundproduced by the transducers so that the phased array can dynamicallyfocus the ultrasound without being moved mechanically. To enable thearray to focus the ultrasound in three dimensions, the transducer sizeshould be sufficiently small to provide an ultrasound field that coversthe entire volume in which the focusing is desired. Decreasing the sizeof the transducers, however, increases their electrical impedance, whichin turn causes mismatch between the RF driving circuitry and thetransducers. Conventional methods for reducing impedance mismatchinclude adding a matching circuit between each transducer and the RFdriving circuitry. As the number of elements within an array increases,supplying a matching circuit for each element becomes costly.

Other conventional techniques for improving electrical-impedancematching include assembling the transducer from multiple layers having acombined electrical impedance that matches the impedance of the RFdriving circuitry. Manufacturing such transducers can be complicated andcostly, and their usefulness for therapeutic applications has not beendemonstrated.

SUMMARY

In an aspect, the invention features a method for manufacturing atransducer component. The method includes adjusting a first dimension ofa piezoelectric component to cause the piezoelectric component to emitsound at the selected frequency; and adjusting a second dimension of thepiezoelectric component to affect an impedance of the piezoelectriccomponent.

In another aspect, the invention features a system including atransducer configured to emit ultrasound in response to receiving anelectrical signal from a driving circuit. The transducer has a firstdimension that determines a frequency of the ultrasound; and a seconddimension that determines an impedance of the transducer, wherein thefrequency is independent of the second dimension.

In a further aspect, the invention features an ultrasound arrayincluding a plurality of transducers each configured to emit ultrasoundin response to receiving an electrical signal from a driving circuit.Each of the plurality of transducers has a first dimension thatdetermines a frequency of the ultrasound; and a second dimension thatdetermines an impedance of the transducer, where the frequency isindependent of the second dimension. The ultrasound array also includesa substrate attached to the plurality of transducers.

Embodiments may include one or more of the following. The impedance ofthe transducer may be independent of the first dimension. The transducermay include a piezoelectric cylindrical wall having an inner surface andan outer surface concentric with the inner surface, where the innersurface defines a lumen. The first dimension may depend on a length ofthe cylindrical wall, and the second dimension may depend on a distancebetween the inner and outer surfaces. The transducer may also include afirst electrode located along the inner surface; a second electrodelocated along the outer surface; and a loading material (e.g., water,air, silicone, and epoxy) inserted within the lumen, where the loadingmaterial affects the impedance of the transducer. A first electricalconductor may be attached to the first electrode; and a secondelectrical conductor may be attached to the second electrode.Furthermore, the first and second electrical conductors may beconfigured to transmit the electrical signal from RF-driving circuitryto the first and second electrodes. The piezoelectric cylindrical wallmay include multiple portions having different lengths and configured toemit ultrasound having multiple frequencies determined by the differentlengths of the multiple portions. The loading material may include acombination of different materials (e.g., air and water). The frequencymay range between 50 kHz and 10 MHz

The impedance prior to adjusting the first dimension may besubstantially equal to the impedance after adjusting the firstdimension. Adjusting the first dimension of the transducer may includeadjusting a length of the cylindrical wall (e.g., by cutting thecylindrical wall along a cross section); and adjusting the seconddimension of the transducer may include adjusting a distance between theinner and outer surfaces. For example, the length of the cylindricalwall may be adjusted after the distance between the inner and outersurfaces has been adjusted. Manufacturing the transducer component mayalso include forming a first electrode along the inner surface; forminga second electrode along the outer surface; and inserting a loadingmaterial within the lumen (e.g., partially filling the lumen with theloading material). The transducer may be mounted on a substrate of anultrasound phased array.

The transducers of the ultrasound array may be arranged such that theircenter-to-center spacings are equal to at most one half of a wavelengthcorresponding to the frequency. The substrate may be planar or have aspherical curvature. The plurality of transducers may be configured toemit first ultrasound radiation having a power level for ablating tissueand second ultrasound radiation having a power level for imaging tissue.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1a shows an ultrasound phased array of transducers;

FIG. 1b shows a transducer for use with the ultrasound phased array ofFIG. 1 a;

FIGS. 2a-2b show a flowchart of a process for fabricating the ultrasoundarray shown in FIG. 1 a.

FIGS. 3-4 show plots of the impedance and phase curves of transducers ofthe type shown in FIG. 1 b;

FIG. 5 shows a plot of the impedance and phase curves of transducers ofthe type shown in FIG. 1b with different loading conditions;

FIG. 6 shows a plot of the impedance, phase, and acoustic outputefficiency curves of a transducer of the type shown in FIG. 1 b;

FIG. 7 shows a plot of the acoustic power as a function of the drivingradio-frequency (RF) power for a transducer of the type shown in FIG. 1b; and

FIGS. 8-9 show contour plots of ultrasound fields produced bytransducers of the type shown in FIG. 1 b.

DETAILED DESCRIPTION

Manufacturing the transducers such that their electrical impedanceclosely matches the output impedance of RF driving circuitry eliminatesthe need for additional matching circuitry and reduces impedancemismatch. As used herein, “impedance” refers to electrical impedanceunless otherwise indicated.

The invention described herein is directed to an apparatus that includesan ultrasound transducer whose impedance can be fine-tuned duringmanufacturing. A first physical dimension of the transducer determinesits impedance and a second physical dimension of the transducerdetermines the frequency of the ultrasound emitted by the transducer.Adjusting the first physical dimension has little to no effect on thefrequency of the ultrasound and adjusting the second physical dimensionhas little to no affect on the impedance of the transducer. Thus, thestructure of the transducer enables the impedance of the transducer andthe frequency of the ultrasound to be controlled independently.

FIG. 1a shows an example of an ultrasound phased array 10 with impedancematching. The phased array 10 includes identical transducers 12 a-d thatare mounted to a substrate 26, which is typically a plastic, such asplexiglass. Although the array 10 in FIG. 1a is shown to have only fourtransducers 12 a-d, in practice, the array 10 includes more than fourtransducers. In some embodiments, the array 10 includes overone-thousand transducers. To provide for focusing everywhere in front ofthe phased array 10, the center-to-center spacing between thetransducers is equal to or smaller than one half wavelength of theemitted ultrasound. Larger spacing may be used when the range of focusis limited and/or when the substrate 26 is non-planar. For example, aspherically curved array can have a larger spacing. Any number oftransducers could be arranged in any pattern. For example, onearrangement has a thousand transducers arranged in concentric circles inwhich the horizontal distances and vertical distances between thecenters of the transducers are equal to half of the wavelength of theultrasound. The vertical and horizontal spacing between the centers ofthe transducers could be equal but different from one half wavelength ofthe ultrasound, or they could be unequal. Other patterns could include agrid, a spiral, or an irregular pattern, though any pattern is possible.The array 10 could include transducers of the same size, shape, andmaterial composition, or the array 10 could include any combination oftransducers of different sizes, shapes, and material compositions. Thesubstrate 26 of the array can be other than planar. For example, thesubstrate 26 can have a spherical curvature.

In some embodiments, the ultrasound phased array 10 is used fortherapeutic applications (e.g., tissue ablation). In other embodiments,the ultrasound phased array 10 is used for diagnostic purposes (e.g.,imaging tissue).

FIG. 1b shows an example of a transducer 12, which could be any of thetransducers 12 a-d, for use in the ultrasound phased array 10. Thetransducer 12 is mounted to the substrate 26. The transducer 12 includesa cylindrical wall 14 composed of piezoelectric material. Thecylindrical wall 14 includes concentric inner and outer surfaces thattogether define a hollow cylinder in which the inner surface defines alumen (i.e., the hollow space within the cylinder). A first conductivematerial (for example metal) applied along the inner surface of thecylindrical wall 14 forms an inner electrode 16 and a second conductivematerial (for example metal) applied along the outer surface of thecylindrical wall 14 forms an outer electrode 18. The conductivematerials are composed for example of metals or alloys, or conductiveepoxies or paints. Examples of electrode metals include copper, nickel,gold, and silver.

Inner and outer conductors (for example wires, or metal foils) 20 and 21are connected (for example soldered) to the inner and to the outerelectrodes 16 and 18, respectively. The conductors 20 and 21 provideelectrical connections to RF driving circuitry (not shown). In someembodiments, the conductors 20 and 21 are composed of copper or silver.In some embodiments, the ends of the conductors include connectors thatengage the RF driving circuitry.

A non-conductive layer 22 interposed between the cylindrical wall 14 andthe substrate 26 provides a low impedance backing on the back end of thetransducer and thus reduces wave propagation into the substrate andmaximizes the power output from the front end of the transducer 12. Inother embodiments, the non-conductive layer 22 provides a high-impedancebacking. One example of a non-conductive layer 22 having a low acousticimpedance is a 2 mm thick cork layer. In some embodiments, a layer ofglue holds the non-conductive layer 22 to the substrate, and a layer ofsilicone rubber holds the cylindrical wall 14 to the non-conductivelayer 22. In other embodiments the acoustic impedance of the backingmaterial is selected to maximize the wave transmission into the backingto provide a wide-band response (also referred to as a short-pulseresponse).

A loading material 24 fills the lumen defined by the inner electrode 16.Examples of a loading material 24 include liquids (such as water), gases(such as air), or solids (such as silicone, and epoxy). The loadingmaterial 24 can also be a mixture or two or more materials, for example,tungsten powder loaded with epoxy. The loading material 24 may also becontrollable during operation. This allows the impedance of thetransducer 12 to be changed while the transducer 12 is operating, forexample, by configuring the transducer 12 such that a controllableamount of fluid can be pumped into the lumen of the transducer 12 whilea portion of the lumen is filled with another fluid. The loadingmaterial 24 is selected to adjust the impedance of the ultrasoundtransducer 12. In some embodiments, the loading material 24 includes acombination of materials, such as air and water. In other embodiments, amembrane such as a polyvinylchloride (PVC) membrane covers the end ofthe transducer 12 to hold the loading material 24 inside the lumen.

The piezoelectric material of the cylindrical wall 14 is polarized in adirection along the radial axis of the cylindrical wall 14. Examples ofpiezoelectric materials include piezo-ceramic materials, such as leadzirconate titanate (PZT), calcium-modified PbTiO₃ (PCT), and SrBi₂Ta₂O₉(SBT). In some embodiments, the piezoelectric material is a compositematerial and/or a material that includes polymers. The cylindrical wall14 is manufactured by conventional techniques, such as those used by EDOElectro-Ceramic Products of Salt Lake City, Utah, and Valpey-Fisher ofHopkinton, Mass. The cylindrical wall 14 can also be formed by injectionmolding, using micro machining, or using nanotechnology methods or anyother methods.

When driven by the RF circuitry, the transducer 12 produces ultrasoundhaving a frequency that corresponds to the vertical length of thecylindrical wall 14. In particular, the length of the cylindrical wall14 determines the transducer's natural frequency of vibration. Thisfrequency is referred to as the “length-mode resonant frequency”. Whendriven by an RF signal having a frequency equal to its length-moderesonant frequency, the transducer 12 absorbs more oscillatory energy.This causes it to vibrate with greater amplitude. The vibrations of thetransducer 12 form an ultrasound wave that propagates from its exposedend 28.

The surface area of the inner and the outer electrodes 16 and 18, thethickness of the cylindrical wall 14, and the loading material 24,determine the impedance of the transducer 12. For a given loadingmaterial 24, the impedance of the transducer is directly proportional tothe wall thickness and inversely proportional to the electrode surfacearea. The cylindrical design of the transducer 12 increases the surfacearea of the inner and the outer electrodes 16 and 18 and reduces thewall thickness to provide overall reduced impedance. The thickness ofthe cylindrical wall 14 can be modified without changing its length.Therefore, adjusting the wall thickness controls the electricalimpedance of the transducer 12 without substantially affecting thefrequency of the ultrasound generated by the transducer 12.

The length of the cylindrical wall 14 ranges between approximately 0.2mm to 30 mm; the outer diameter of the cylindrical wall 14 rangesbetween approximately 0.075 mm and 20 mm; and the thickness of thecylindrical wall ranges between approximately 0.001 mm and 5 mm. Thetransducer 12 can produce ultrasound having a frequency that rangesbetween approximately 50 kHz and 10 MHz. In some embodiments, thefrequency of the ultrasound produced by transducer 12 is greater than 10MHz. The impedance of the transducer 12 can be adjusted to match theimpedance of the RF driving circuitry, which is typically a standard 50ohms or 100 ohms.

FIGS. 2a-2b show a process 40 for constructing the ultrasound array 10shown in FIG. 1a . The impedance and frequency specifications of eachtransducer 12 in the ultrasound array 10 are determined and stored, forexample, in a computer (step 42). From the impedance specification, thethickness and outer diameter of the cylindrical wall 14 is determined(step 44). In some embodiments, the thickness and outer diameter aredetermined using a processor that calculates these dimensions using oneor more mathematical equations. In other embodiments, the thickness anddiameter are determined from a lookup table, stored in memory, whoseentries contain thickness and outer diameter values corresponding toimpedances. From the frequency specification, the length of thecylindrical wall is determined (step 46). In some embodiments, aprocessor calculates the length using one or more mathematicalequations. In other embodiments, a processor retrieves the length from alookup table, stored in memory, whose entries contain lengthscorresponding to frequencies. The values stored in the lookup table maybe obtained empirically or by calculation.

The cylindrical wall 14 is then fabricated to have the determined outerdiameter, wall thickness, and length (step 48). In some embodiments, thecylindrical wall 14 is first constructed according to the thickness anddiameter specifications and then cut to a desired length using, forexample, a diamond wire saw. The inner and outer electrodes 16 and 18are formed as metal coatings on the respective inner and outer surfacesof the cylindrical wall 14 (step 50). In some embodiments, steps 48 and50 are performed in reverse order (i.e., the top and bottom surfaces ofa rectangular sheet of piezoelectric material are coated with metal andthen the sheet is curled to form the cylindrical wall 14 with the innerand outer electrodes 16 and 18 already attached). If more transducersneed to be fabricated (step 52), the steps 42, 44, 46, 48, and 50 arerepeated. Otherwise, the transducers are mounted one-by-one onto thesubstrate 26. The non-conductive layer 22 is mounted on the substrate(step 54). In some embodiments, the non-conductive layer 22 is glued tothe substrate. The cylindrical wall 14 is attached to the non-conductivelayer 22 (step 56). In some embodiments, the cylindrical wall 14 ismounted on the non-conductive layer 22 with a layer of silicone rubber.The wires 20 and 21 are soldered onto the inner and outer electrodes 16and 18 to provide electrical connections to RF circuitry (step 58). Thelumen defined by the inner electrode 16 is filled with a loadingmaterial 24 (step 60) selected to adjust the impedance of the ultrasoundtransducer 12. In some embodiments, the loading material 24 is chosenfrom a group consisting of water, air, silicone, and epoxy. In someembodiments, a thin membrane is attached at the end of the cylindricalwall 14 to provide air loading in and around the transducer 12. In otherembodiments, the loading material 24 is a combination of materials(e.g., air and water). In further embodiments, the loading material 24only partially fills the lumen defined by the inner electrode 16. Afterthe transducer 12 has been mounted, a determination is made as towhether all of the transducers have been mounted to the phased array 10.If there are transducers remaining, the process 40 repeats steps 54, 56,58, and 60 until all of the transducers are mounted.

The steps of the process 40 are not limited to the sequence shown andcould be performed in other sequences. For example, a transducer 12could be fabricated (steps 42, 44, 46, 48, and 50) and then immediatelymounted to the phased array 11 (steps 54, 56, 58, and 60) before anothertransducer is fabricated. In some embodiments, multiple transducers aremounted in parallel. In other embodiments the array 11 is constructed byinjection molding or by using micro machining or nanotechnology methods.

Examples

The feasibility of using the transducer 12 and the ultrasound phasedarray 10 shown in FIG. 1a-1b for therapeutic and diagnostic applicationswas tested. All of the transducers under test had the same structure asthe transducer 12 shown in FIG. 1b . The transducers were driven by RFsignals that were produced by a frequency generator (Model DS345 orWavetek 395, available from Stanford Research Systems of Sunnyvale,Calif.) and amplified with a RF-amplifier (Model 2400L, available fromENI, Inc. of Rochester, N.Y.). The electrical impedance of thetransducers was measured using a network analyzer (Hewlett-Packard,Model 4195A).

FIG. 3 shows plots 70 and 72 of impedance and phase curves that weremeasured for two transducers having the same outer diameter (10 mm) andwall-thickness (1.3 mm) but different lengths (4.8 mm and 10 mm). Forease of explanation, the transducer with the length of 4.8 mm will bereferred to as “transducer T1” and the transducer with the length of 10mm will be referred to as “transducer T2”. The dotted curve correspondsto transducer T1 and the solid curve corresponds to transducer T2. Bothtransducers T1 and T2 exhibited a thickness-mode resonant frequency of1.5 MHz and were loaded with air. The symbol “L” in FIGS. 3-4 denotesthe length-mode resonant frequency and the symbol “L3rd” denotes thethird harmonic of the length-mode resonant frequency. The plot 72 of thephase curves indicates that the transducer T2 exhibited a fundamentallength-mode resonant frequency at approximately 175 kHz and a thirdharmonic length-mode resonant frequency at approximately 500 kHz andthat the transducer T1 exhibited a fundamental length-mode resonantfrequency of approximately 350 kHz and a third harmonic length-moderesonant frequency of approximately 900 kHz.

FIG. 4 shows plots 80, 82, 84, and 86 of impedance and phase curves thatwere measured for two transducers having the same diameters and lengthsof 15 mm and 25 mm, respectively, but different cylindrical-wallthicknesses of 4 mm and 2 mm. For ease of explanation, the transducerhaving the cylindrical-wall thickness of 4 mm is referred to as“transducer T3” and the transducer having the cylindrical-wall thicknessof 2 mm is referred to as “transducer T4”. A thickness-mode resonantfrequency of approximately 0.5 MHz was measured for the transducer T3while a thickness-mode resonant frequency of approximately 1.0 MHz wasmeasured for the transducer T4. The impedance of the transducer T3exhibited a minimum impedance of 642 Ohms and the impedance of thetransducer T4 exhibited a minimum impedance of 327 Ohms. The transducersT3 and T4 both exhibited a length-mode resonant frequency ofapproximately 50 kHz. Another, non-overlapping resonant frequency,denoted in FIG. 4 by the symbol “TA”, was observed at 100 kHz for thetransducer T3 and at approximately 85 kHz for the transducer T4.

FIG. 5 shows plots 90, 92, 94, and 96 of impedance and phase curves fora transducer having different loading materials (i.e., air, water, andair with the end of the transducer in water). In many of themeasurements, a thin PVC membrane (thickness 0.07 mm) was attached atthe end of the transducer to provide air loading in and around thecylindrical wall. The impedance curves of plots 90, 92, and 94 indicatethat the impedance of the transducer at its length-mode resonancedepended on the loading material. According to the phase curves shown inplot 96, the length-mode resonances (i.e., the frequency of the peakphase) varied only slightly depending on the loading condition. Airloading produced the lowest impedance value at the length-mode resonantfrequency and the highest peak impedance value. Water loading decreasedthe magnitude of the peak impedance but had only a small impact on thephase. Air loading with the end of the transducer touching the waterproduced impedance and phase curves that lay in between the air-loadingand water-loading curves.

The outer-diameter dimensions of the transducers, which were on theorder of a wavelength of the emitted ultrasound, caused the emittedultrasound to diverge. Because the emitted ultrasound was measured afterit had begun to diverge, the actual conversion-efficiency values of manyof the tested transducers were higher than the measuredconversion-efficiency values. The measured conversion-efficiency valueswere determined by dividing the electric power of the RF-driving signalby the acoustic power measured at the output of the transducer.

FIG. 6 shows a plot 104 of conversion-efficiency curves of thetransducer whose impedance and phase curves are shown in the plots 90,92, 94, and 96 in FIG. 5. The plots 102 and 104, also shown in FIG. 6,show the impedance and phase curves for the transducer when it wasloaded with water and when it was loaded with air with its end touchingwater. These impedance and phase curves are identical to those shown inthe plots 90 and 96 for the same loading conditions. According to theconversion-efficiency curves shown in the plot 104, the maximumconversion efficiency of the transducer occurred approximately at thepeak phase for both loading conditions. The conversion efficiency wasrelatively constant between the peak phase and the peak impedance whenjust the tip of the transducer was touching water, whereas when thetransducer was completely filled with water, the acoustic efficiency wasreduced at the frequency corresponding to the peak impedance. However,at the minimum impedance value, the conversion efficiency was reducedonly approximately 30% from the peak efficiency to the minimumimpedance, indicating that the impedance of the transducer 12 can beselected from a wide-range of values without causing the conversionefficiency of the transducer 12 to fall below an adequate level.

To measure the acoustic power and efficiency of a transducer at aselected driving frequency, an external matching network was constructedand tuned for a transducer operating at the selected driving frequency.The matching network was also coupled to RF-power monitoring equipmenthaving a 50-ohm impedance. The RF-power monitoring equipment, whichincluded a dual directional coupler (Model C1737, available fromWerlatone) and a digital power meter (Model 438A, available fromHewlett-Packard), measured the forward and reflected electric power atthe interface between the matching network and the transducer undertest. In a radiation-force measurement system, an absorbing target wasconfigured to receive the ultrasound generated from the end of thetransducer and measure the acoustic power of the ultrasound.

FIG. 7 shows plots 110, 112, and 114 of the acoustic power as a functionof the driving RF power for various transducers under test. The plot 110shows the acoustic-power curve of a transducer when it was driven at itsfundamental length-mode resonant frequency (i.e., 173 kHz) and theacoustic-power curve of the same transducer when it was driven at thethird harmonic of its length-mode resonant frequency (i.e., 474 kHz).The transducer had a length of 10 mm, an outer diameter of 10 mm, and awall thickness of 1.3 mm. An approximately linear relationship wasobserved for each acoustic power curve shown in plot 110.

The plot 112 shows acoustic-power curves that were measured for twotransducers having a length of 5 mm, cylindrical-wall widths 1.3 mm and4 mm, and outer diameters of 10 mm and 15 mm, respectively. For ease ofexplanation, the transducer having a cylindrical-wall width of 1.3 mmand an outer diameter of 10 mm will be referred to as “transducer T5”and the transducer having a cylindrical-wall width of 4 mm and an outerdiameter of 15 mm will be referred to as “transducer T6”. The curveswith circular markers correspond to the transducer T5 and the curveswith square markers correspond to the transducer T6. Curves withdifferent markers correspond to different transducers having wallthicknesses of 1.3 mm. As shown in plot 112, the water-filledtransducers exhibited a slightly higher conversion efficiency than theair-filled transducers and provided higher power outputs. The waterfilled transducers were driven to their limit such that the wireconnections to the electrodes were destroyed. A maximum powermeasurement of 10.7 W and 11.3 W was achieved for transducers T5 and T6before they failed. The transducers T5 and T6 failed at the locationswhere the electrodes were connected to the wires. Higher powers may havebeen achieved if the connections between the electrodes were made morerobust.

Plot 114 shows acoustic-power curves that were measured for thetransducer T5 when it was filled with water and when it was filled withsilicone rubber. A comparison of the two acoustic-power curves showsthat using silicone as a loading material did not significantly reducethe efficiency compared to using water as a loading material.

Contour plots of ultrasound fields produced by transducers under testwere determined using the following procedure. A needle hydrophonehaving a spot diameter of 1 mm detected the pressure wave distributionsof the ultrasound produced by the transducers under test.Computer-controlled stepper motors moved the hydrophone to predeterminedlocations that were stored in the computer. At each of the severalpredetermined locations, the hydrophone measured the pressure wave ofthe ultrasound emitted from the transducer under test. The hydrophoneencoded the pressure wave measurements as electrical signals that werethen amplified using an amplifier (available from Precision AcousticsLtd) and stored by an oscilloscope (model 2431L, available fromTektronix). The oscilloscope measured the amplitudes of the storedpressure waves and the computer stored these amplitude measurements inmemory. From the amplitude measurements and their correspondinglocations, the computer calculated the ultrasound field generated by thetransducer.

FIG. 8 shows contour plots 120, 122, and 124 of ultrasound fieldsproduced by a transducer that was driven with different RF frequenciesand subjected to different loading conditions. The transducer had alength of 10 mm, an outer diameter of 6 mm, and a wall thickness of 0.3mm. The contour plots 120 and 122 show the ultrasound fields produced bythe transducer when it was completely filled with water and driven at RFfrequencies of 343 kHz and 311.5 kHz, which corresponded, respectively,to the frequencies at which the transducers impedance reached itsmaximum and minimum values. As expected, the contour plots 120 and 122show some variation as the frequency increases, but they also indicatethat the shape of the ultrasound beam did not depend significantly onthe minimum and maximum impedance values. The contour plot 124 shows theultrasound field that was produced by an air-loaded transducer coveredby a PVC membrane, a portion of which was in contact with water. Thetransducer was driven at an RF frequency of 311.5 kHz, a frequency atwhich it reached its minimum impedance. As seen in the contour plot 124,the ultrasound field produced by the transducer was slightly moredirected than the ultrasound fields shown in the contour plots 120 and122. Increasing the outer diameter of the transducer also produced amore directed ultrasound field. The ultrasound field directivitydecreased when the outer diameter was decreased and approached that of apoint source as the outer diameter approached half of a wavelength ofthe emitted ultrasound.

Driving the transducers at higher harmonics also produced ultrasoundfields that were more directed than those produced at the fundamentalfrequencies. FIG. 9 shows contour plots 130, 132, and 134 of theultrasound fields produced by a transducer having a length of 10 mm, anouter diameter of 6 mm, and a wall thickness of 0.3 mm. A PVC membrane,a portion of which was in contact with water, provided an air surfacearound the transducer. The contour plot 130 shows the ultrasound fieldproduced by the transducer when it was driven at its fundamentallength-mode resonant frequency (i.e., 192 kHz), a frequency at which thetransducer reached its minimum impedance. The contour plot 132 shows theultrasound field produced by the transducer when it was driven at thethird harmonic of the length-mode resonant frequency (i.e., 465 kHz).The contour plot 134 shows the ultrasound field produced by thetransducer when it was driven at the fifth harmonic of the length-moderesonant frequency (i.e., 778 kHz).

Discussion

The results shown in FIGS. 3-9 demonstrate that it is possible to uselength-mode coupling to produce therapeutically useful ultrasound poweroutputs from ultrasound transducers 12 of the type shown in FIG. 1b .The structure of the transducer 12 allows the transducer impedance to beadjusted to a desired value so that impedance matching can be achievedeven with small transducer-dimensions used in phased arrays with fullwave control.

The power measurements of the transducers under test translated to amaximum transducer surface intensity of 13.6 W/cm2, a value that ishigher than what is currently used for many therapy applications. Thelength-mode resonant frequencies of the transducers under test wereproportional to the lengths of the transducers, and their impedancevalues were directly proportional to their wall thicknesses andinversely proportional to their electrode surface areas. The transducersproduced adequate power as their impedances were adjusted from a minimumvalue to a maximum value. The loading material also had an impact on theimpedance, but had little to no effect on the power output or on theultrasound field.

One therapeutic application of the phased-array 10 is that of performinglow frequency (200-300 kHz) transcranial sonications that are used, forexample, to disrupt the blood-brain barrier or to treat thrombolysis. Asystem designed for this application includes a hemispherical array witha diameter of 25-30 cm. According to the results described in connectionwith FIGS. 3-9, adequate electronic steering of such an array can beachieved with 10 mm diameter transducers. Transducers having the desireddimensions and output frequency range were produced. Some of thesetransducers had diameters that were approximately equal to half of thewavelength of the emitted ultrasound. Thus the practicality of atransducer 12 for this particular application was demonstrated.

OTHER EMBODIMENTS

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the transducers could be configured to resonate at higherfrequencies than the transducers tested in the study.

The lumen of the transducer 12 could be used to provide additionalfunctionality to the array 10. In some embodiments, imaging transducersor hydrophones are placed within the lumen of the transducer 12. Inother embodiments, a detector placed within the lumen of the transducer12 receives diagnostic signals that are emitted by the transducer 12. Infurther embodiments, a transducer of a multi-frequency array is composedof two or more transducers 12 of different lengths that are arrangedconcentrically.

In some embodiments, the transducer 12 is part of an intravascularcatheter that delivers ultrasound. Such a catheter could be useful forthrombolysis treatment, targeted drug delivery, gene therapy, and otherapplications. In some of these embodiments, the frequency produced bythe transducer 12 is lower than the lowest frequencies that aretypically generated by conventional catheters. In some embodiments, thelumen of the transducer 12 includes guide-wires or systems forirrigation and debris removal.

In other embodiments, the cylindrical wall 14 has non-parallel ends thusmaking the length of the wall uneven. This results in multi-frequencyexcitation. The transducer 12 could be arranged to include two or moreconcentric piezoelectric cylinders of the same or different length withinner electrodes and outer electrodes connected together.

In further embodiments, the transducer 12 includes: a piezoelectricplate having an inner surface and an outer surface, in which a the firstdimension of the plate is a length or width of the plate and the seconddimension is a thickness of the plate (e.g., a distance between theinner and outer surface). The inner surface of the plate includes afirst electrode, and the outer surface of the plate includes a secondelectrode. A loading material that affects the impedance of thetransducer may be inserted around the plate.

In some embodiments, the transducer 12 includes two or more parallelplates of equal size and shape with first and second electrodes locatedon the inner and outer surface of each plate. The impedance of thetransducer is determined by the thickness and number of the plates andthe resonance frequency is determined by the length or width of theplates. The first and second electrodes of each plate are electricallyconnected. A loading material that affects the impedance of thetransducer may be inserted between the plates. To enable the transducer12 to be excited at multiple frequencies, a dimension (e.g., length orwidth) of one or more of the plates is selected to be uneven (i.e., tohave different lengths).

In other embodiments, the transducer 12 includes a piezoelectricmaterial with electrodes on surfaces of structures within the transducer12. The piezoelectric material emits ultrasound in response to receivinga driving signal having a frequency equal to the resonance frequency ofthe transducer 12. The acoustic impedance of the transducer 12 isdetermined by thickness and size of the piezoelectric material betweenthe electrodes and the resonance frequency of the transducer 12 isdetermined by the dimension of the material parallel with theelectrodes.

Accordingly, other embodiments are within the scope of the followingclaims.

1-20. (canceled)
 21. An ultrasound system comprising: an ultrasoundtransducer comprising: a piezoelectric material characterized by alength and a thickness, wherein said piezoelectric material comprises afirst side surface and a second side surface defining said thickness; afirst electrode and a second electrode respectively contacting saidfirst side surface and said second side surface of said piezoelectricmaterial; and a loading material contacting at least one of said firstelectrode and said second side surface; and driving circuitry inelectrical communication with said first electrode and said secondelectrode, said driving circuitry having an output electrical impedanceassociated therewith; wherein said driving circuitry is configured toapply driving signals to said ultrasound transducer with a frequencyselected to excite a length-mode resonance via length-mode coupling,such said ultrasound transducer emits ultrasound energy along a lengthdirection associated with said length; and wherein one or more of acomposition of said loading material and a quantity of said loadingmaterial is selected such that a transducer electrical impedanceassociated with said length-mode resonance is selected to match saidoutput electrical impedance of said driving circuitry.
 22. Theultrasound system according to claim 21 wherein said piezoelectricmaterial is formed as a cylinder, and wherein said first electrode isprovided on an inner surface of said cylinder, and said second electrodeis formed on an outer surface of said cylinder.
 23. The ultrasoundsystem according to claim 22 wherein said loading material is providedwithin a lumen of said cylinder.
 24. The ultrasound system according toclaim 23 wherein said loading material partially fills said lumen. 25.The ultrasound system according to any one of claim 21 wherein saidpiezoelectric material is formed as a plate.
 26. The ultrasound systemaccording to claim 25 wherein said loading material is provided aroundsaid plate.
 27. The ultrasound system according to any one of claim 21wherein said loading material comprises one or more of air, water,silicone and epoxy.
 28. The ultrasound system according to any one ofclaim 21 further comprising a membrane to secure said loading materialin place relative to said ultrasound transducer.
 29. The ultrasoundsystem according to any one of claim 21 further comprising a pluralityof said ultrasound transducers, wherein said plurality of ultrasoundtransducers are arranged and controlled in a phased array.
 30. A methodof performing impedance matching of an ultrasound system, the ultrasoundsystem comprising: an ultrasound transducer comprising: a piezoelectricmaterial characterized by a length and a thickness, wherein saidpiezoelectric material comprises a first side surface and a second sidesurface defining said thickness; and a first electrode and a secondelectrode respectively contacting said first side surface and saidsecond side surface of said piezoelectric material; and drivingcircuitry in electrical communication with said first electrode and saidsecond electrode, said driving circuitry having an output electricalimpedance associated therewith; and contacting at least one of saidfirst electrode and said second electrode with a loading material,wherein one or more of a composition and a quantity of said loadingmaterial is selected such that a transducer electrical impedanceassociated with excitation, via length-mode coupling, of a length-moderesonance, is selected to match said output electrical impedance of saiddriving circuitry, wherein said length-mode resonance is associatedemission of ultrasound energy along a length direction.
 31. The methodaccording to claim 30 wherein said piezoelectric material is formed as acylinder, and wherein said first electrode is provided on an innersurface of said cylinder, and said second electrode is formed on anouter surface of said cylinder.
 32. The method according to claim 31wherein said loading material is provided within a lumen of saidcylinder.
 33. The method according to claim 32 wherein said loadingmaterial partially fills said lumen.
 34. The method according to any oneof claim 30 wherein said piezoelectric material is formed as a plate.35. The method according to claim 34 wherein said loading material isprovided around said plate.
 36. The method according to any one of claim30 wherein said loading material comprises one or more of air, water,silicone and epoxy.
 37. The method according to any one of claim 30further comprising providing a membrane to secure said loading materialin place relative to said ultrasound transducer.
 38. The methodaccording to any one of claim 30 further comprising providing additionalultrasound transducers to form a phased array.
 39. The method accordingto any one of claim 30 further comprising applying driving signals tosaid ultrasound transducer with a frequency selected to excite saidlength-mode resonance via length-mode coupling, such said ultrasoundtransducer emits ultrasound energy along said length direction.